RAINFALL MEASUREMENT BY RADAR - A REVIEW

Several measuring methods using various properties of radar signals to estimate rainfall rate R are reviewed. These methods are based on mea surements of the radar reflectivity factor Z at one or two polarizatio ns, on measurements of attenuation A or on a combination of these two quantities. When only one radar observable is measured, R is computed from analytical relations of R-Z or A-Z type, ignoring the spatiotempo ral variations of the raindrop size distribution N(D), where D is the drop diameter. If N(D) can be approximated by a two parameter equation , R can be estimated from the determination of N(D), using measurement s of two radar observables. Some of these methods are notably improved if the radar data are combined with rain gauge network measurements a t the ground. The mean volume rainfall and the areally averaged rain r ate can be estimated from the area-time integral and fractional area m ethods.